.alpha.-Alumina powder is conventionally produced by the Bayer process, which comprises neutralizing sodium aluminate with an acid to give aluminum hydroxide and calcining it at an elevated temperature of 1200.degree. C. or higher. Since the obtained alumina has an uneven particle size and a specific surface area of smaller than 10 m.sup.2 /g and since sintering of particles occurs during the calcining, fine .alpha.-alumina powder can not be obtained. There is another process known for producing .alpha.-alumina powder, which comprises hydrolyzing aluminum alkoxide, e.g. aluminum isopropoxide, to give aluminum hydroxide and heating it at a high temperature. The obtained .alpha.-alumina powder has a surface area of less than 10 m.sup.2 /g, that is, desired fine .alpha.-alumina powder can not be obtained.
The abrasiveness or grinding property of alumina particles is exhibited by high hardness of .alpha.-alumina free of .gamma.-alumina. Owing to its low hardness, .gamma.-alumina can not be used as an abrasive material. In the present invention, the term .alpha.-alumina is used as a general term including .delta.-, .gamma.-, .theta.- and .kappa.-alumina, which are produced at low temperatures and which are determined by X-ray diffraction analysis. To have superior abrasiveness, .alpha.-Alumina particles are required to have high crystallinity in addition to being .alpha.-alumina. Therefore, even if particles are recognized by X-ray diffraction as being composed of .alpha.-alumina alone, they are poor in abrasiveness unless the .alpha.-alumina has a high X-ray diffraction intensity. Preferably, they should have an X-ray diffraction intensity of at least 7000 cps at mirror plane index (113). By conventional manufacturing processes, it was difficult to produce fine .alpha.-alumina powder having a specific surface area greater than 10 m.sup.2 /g and a high crystallinity in terms of X-ray diffraction intensity of at least 7000 cps.
The production of .alpha.-alumina powder from .gamma.-alumina powder or aluminum hydroxide powder usually involves calcination of the starting material powder at 1150.degree.-1300 .degree. C. In general, calcination at a higher temperature is necessary to yield a powder having a high crystallinity. However, calcination at a higher temperature causes powder particles to sinter forming coarse particles. Therefore, calcination at a lower temperature is desirable to yield a fine powder. In this case, however, the resulting .alpha.-alumina powder has a low crystallinity.
For example, calcination of aluminum hydroxide powder at about 1150.degree. C. gives a fine .alpha.-alumina powder having a specific surface area of about 30 m.sup.2 /g. However, this powder has a low crystallinity in terms of X-ray diffraction intensity of about 6000 cps, and it also contains .gamma.-alumina other than .alpha.-alumina in a large quantity. On the other hand, the calcination temperature of about 1200.degree. C. increases the crystallinity of the resulting alumina to about 11550 cps in the X-ray diffraction intensity but decreases the specific surface area to about 7.0 m.sup.2 /g.
Prior to the present invention, the present inventors found that it is possible to produce a fine .alpha.-alumina powder having a high crystallinity, if a .gamma.-alumina powder obtained by the dry process is calcined together with a small amount of fine silica powder. A production process based on this finding is the subject of Japanese Patent Application No. 219860/1989. Continued research led to a finding that calcining aluminum hydroxide containing fine silica powder uniformly dispersed therein gives a fine .alpha.-alumina powder having a high crystallinity. Especially, it was found that aluminum hydroxide precipitate obtained by hydrolysis of aluminum chloride and the like in the presence of silica powder uniformly dispersed in the solution produces fine .alpha.-alumina powder having a high crystallinity by calcination. The thus obtained alumina powder has a better dispersibility in water than the alumina obtained by calcination of .gamma.-alumina.